A power amplifier is a key component in a wireless transmitter; it is generally the last stage of amplification of a signal before transmission and accordingly it is required to handle potentially high powers, typically several Watts in a cellular wireless transmitter, for example. While other electronic components in a wireless transmit chain have been subject to integration with accompanying reductions in size and cost, the power amplifier has generally remained a bulky and relatively expensive component, due largely to its power dissipation and the need for cooling; it also requires a large and potentially costly power supply, which places heavy demands on battery back-up systems. As a result, there has been much effort directed at the development of more efficient power amplifier designs, aimed at reducing the power dissipation and the associated costs and environmental impact. Generally, efficiency in a power amplifier is achieved with the penalty of an inherently non-linear signal transfer characteristic, and furthermore the non-linearity may be a complex function involving temperature dependence and memory of preceding signal characteristics.
The non-linear characteristics of a power amplifier may be corrected by a pre-distorter, which is a circuit that is controlled in such a way that it introduces distortions to a signal, before it enters an amplifier, that are designed to counteract the effects of the non-linear transfer characteristic of the power amplifier, resulting ideally in an overall linear response for the transmit chain from the input to the pre-distorter to the output of the power amplifier. While it is also possible to introduce corrections to a signal after the power amplifier, it is generally preferable to introduce corrections before amplification, since this may be done at lower power and typically in the digital domain.
FIG. 1 shows a typical transmit chain having a pre-distorter, in a so-called error signal architecture. An input signal 2, typically at baseband, passes through pre-distorter 4 which introduces distortions to the signal, due to the transfer characteristic of the pre-distorter. Typically, the transfer characteristic may be defined by a polynomial, which may include delay terms, and may for example, be a Volterra series. The pre-distorter may be implemented as a finite impulse response digital filter, or alternatively as an infinite impulse response digital filter. It is convenient to implement the pre-distorter in the digital domain at baseband in inphase and quadrature components, but alternatively, the pre-distorter may be implemented at an intermediate frequency or at radio frequency in analogue or digital domains. In the example of FIG. 1, an upconverter 6 is shown which mixes the signal up in frequency before amplification in the power amplifier, but it will be understood that this stage, and the corresponding downconverter 16, is optional, depending on the frequency plan.
Following the pre-distorter and upconverter, the signal is amplified by the power amplifier 8, operating typically at radio frequency. The output signal 10 from the power amplifier passes through a coupler 12 that passes most of the signal to the output of the transmit chain 14 and couples off a proportion of the signal for use in an observation receiver, comprising a downconverter 16 and a sampler 18, typically operating at baseband. Samples are also taken of the input signal by another sampler 20. The pre-distort trainer 22 takes input samples and corresponding samples from the observation receiver (downconverter 16 and sampler 18), that may be referred to as output samples, to control the coefficients of the pre-distorter.
FIG. 2 illustrates the operation of the pre-distort trainer 22 of FIG. 1. Input samples and corresponding output samples are aligned in phase by an aligner and comparator functional block 34 to compensate for the phase characteristics of circuit components and are compared, generating an error signal. The error signal is the difference between the output signal and the input signal, as represented by the samples, and the aligned input signal, used as a reference signal, as represented by input samples. Coefficients of a polynomial are trained in coefficient trainer 36 so that typically the reference, operated by the polynomial, produces an output that would cancel the error signal. The coefficients generated by this process constitute the update coefficients for the pre-distorter, that is to say incremental updates of the coefficients applied by the pre-distorter. The pre-distorter update coefficients are held in a store 38 for use in updating the coefficients applied by the pre-distorter. In a variation of this scheme, the error signal may be generated in the analogue domain before sampling by the observation receiver.
FIG. 3 illustrates an alternative architecture of a transmit chain incorporating a pre-distorter, referred to as a full signal architecture. The system of FIG. 3 differs from the error signal architecture of FIG. 1 in that samples are taken of the output of the pre-distorter, rather than the input signal, for use in the pre-distort trainer. Training of the pre-distorter coefficients is typically performed to produce the result that the sampled output signal of the pre-distorter, acted upon by the polynomial, reproduces the output signal of the amplifier. The coefficients of the polynomial are thus the inverse of the characteristic required of the pre-distorter to counteract the non-linearity. The coefficients to be applied by the pre-distorter can then be derived by an appropriate transformation applied to the trained coefficients. It can be seen that the architecture of FIG. 3 does not calculate an error signal, and trains the whole polynomial coefficients rather than an incremental update of the polynomial coefficients.
The architecture of both FIG. 1 and FIG. 3 are intended for situations in which signal conditions are relatively stable, such as would be expected when the input signal represents a single wireless channel. The frequency band amplified by the amplifier is expected to be fully occupied, or at least occupied by a consistent spectrum, and the amplitude of the signal to be transmitted is expected to be relatively stable over time. In particular, a single relatively static signal is expected, not frequency hopping or varying in amplitude.
However, if multiple channels are transmitted by the amplifier, for example where the channels implement different wireless standards, the assumptions of stability of the frequency and amplitude composition of the aggregate signals may not be valid. In these circumstances, the training of the coefficients may not be stable, and the trained coefficients may not be optimal if the aggregate signal characteristics of the signals in the multiple channels change.
The present invention addresses these disadvantages.